1. Field of the Invention
The present invention relates to a control device for an automatic transmission, and particularly to slip control of a clutch or brake at a disengaged side when shifting up.
2. Description of the Related Art
Conventionally, automatic transmissions have been widely used as transmissions for automobiles etc. With these automatic transmissions, a turbine of a torque converter is made to rotate with the drive shaft of a prime mover such as an engine as input, and this rotation is converted to a specified gear ratio and conveyed to an output shaft by a planetary gear connected to a turbine shaft. A plurality of clutch or brake friction engagement devices are then provided between the turbine shaft and the output shaft in order to regulate movement of this planetary gear, and a gear ratio is switched by which friction engagement device is engaged from these plurality of friction engagement devices. Normally, the friction engagement device is switched according to input shaft rotation speed and extent of pressing on the accelerator to switch gear ratio.
Here, during a gear shift operation, a disengagement side engagement device that is in an engaged state is disengaged and an engagement side engagement device that is selected and in a disengaged state is engaged. Tightening force of these engagement devices is controlled hydraulically, with hydraulic pressure supplied to the disengagement side engagement device being gradually reduced, and hydraulic pressure supplied to the engagement side engagement device being gradually increased to shift gear.
At this time, if the time when hydraulic pressure supplied to the engagement side engagement device is increased is delayed compared to the time when hydraulic pressure supplied to the disengagement side engagement device is reduced, input torque becomes larger than the torque transmission capacity of the disengagement side engagement device and the engagement side engagement device, and the turbine rotation speed suddenly increases. On the other hand, if the time when hydraulic pressure supplied to the engagement side engagement device is increased is advanced compared to the time when hydraulic pressure supplied to the disengagement side engagement device is reduced, transmitted torque of the disengagement side engagement device and the engagement side disengagement device becomes larger than the input torque and the turbine rotation speed drops. This type of situation causes gear shift shock.
A technique has therefore been proposed to allow a disengagement side engagement device to slip by a specified amount before commencing inertia phase control in order to suppress gear shift shock. By allowing a disengagement side engagement device to slip by a specified amount, even if the time when the hydraulic pressure supplied to the engagement side engagement device is increased is early, this is absorbed by the slip of the disengagement side engagement device and gear shift shock is suppressed.
Japanese Patent Laid-open No. 2000-97324 discloses a control device for an automatic transmission for carrying out slip control of a disengagement side engagement device when shifting up by controlling hydraulic pressure supplied to a disengagement side engagement device and an engagement side engagement device. In this related art automatic transmission controller, hydraulic pressure supplied to one of either a disengagement side or an engagement side engagement device, for example an engagement side engagement device, is increased in line with a specified pattern. In this case, a target turbine rotation speed is set in order to cause specified slip in the disengagement side engagement device, and hydraulic pressure supplied to the disengagement side engagement device is controlled based on target turbine rotation speed and a gear shift model.
However, in this related art automatic transmission controller, in order to carry out slip control for the disengagement side engagement device, it is necessary to have slip amount responsiveness and precision with respect to a target value in the case of noise occurring, such as, for example, turbine torque variation or variation in coefficient of dynamic friction of the disengagement side engagement device, and there are problems such as not obtaining a sufficiently improved effect for gear shift shock and an increased time required for the gear shift operation.